Among the range of various different types of technologies that exist in modern industry, one that is pervasive throughout a variety of technological disciplines, and that is important in the manufacture or use of a vast number of commercial and consumer products, is the technology of mixing liquids. Most industries have some level of need for combining and mixing liquid materials, for example to provide homogeneity in a liquid raw material, ingredient, or product. Mixing processes are essential to the food industry, the pharmaceutical industry, to chemical and chemical processing industries, to semiconductor processing and fabrication industries, for preparing and using agricultural chemicals, and in the manufacture of products ranging from chemical materials (e.g., paints, coatings, adhesives, etc.), to polymers (e.g., plastics, thermoplastics, thermosetting polymers, curable monomers and polymers, etc.), building materials, and for use in drilling and mining processes, among others. Liquids that require mixing to homogeneity can be in the form of aqueous or organic-solvent-based solutions, suspensions, emulsions, high solids-containing liquid monomeric or polymeric materials, etc., and may contain an aqueous or organic liquid medium as a solvent of dissolved materials, or as a medium to suspend another solid or liquid material. The liquid or constituent of the liquid may be reactive, curable, or biologically active, etc.
Mixing a liquid, however, must in many instances be performed with specific care to provide a desired mixing effect such as homogeneity, without otherwise undesirably affecting the quality of the liquid being mixed. Certain undesired mixing effects are types that can be easily observed in a liquid during mixing, such as when unwanted foam or bubbles are produced, if a high amount of mixing shear causes coagulation of materials of a liquid, or if mixing produces a vortex that pulls air into the liquid. In other instances, an undesired mixing effect may be less noticeable or possibly considered inconsequential, such as when small droplets of liquid, or residue of a foam, contacts a container sidewall during mixing in a manner that causes solid or dissolved materials of the liquid to become located and retained on the sidewall and removed from the bulk volume of the liquid being mixed.
Certain general and specialty types of mixing technologies have been developed for different areas of industry. In the industries for processing microelectronic devices, specifically with respect to chemical-mechanical polishing (CMP) processes, liquid compositions referred to as slurries (“CMP slurries”) contain water, suspended abrasives, and dissolved chemicals. Other liquid compositions useful in the semiconductor and microelectronics industries contain dissolved chemicals such as acid, base, surfactant, polymers, and other chemicals. These liquids are used in extremely sensitive and well-controlled processes of polishing, planarizing, or cleaning in-process microelectronic and semiconductor devices. The chemical ingredients are present at very low concentrations, sometimes in a range of a number of parts per million, and the concentration of each chemical ingredient must remain at a set concentration for the ingredient, to achieve consistent performance upon use of the liquid.
During shipment or storage of a CMP slurry that contains suspended particles, the suspended particles can settle at the bottom of a container, or (less obviously) can become stratified due to gravity within the liquid, meaning that liquid at a lower portion of the container contains a higher concentration of suspended particles, and liquid at the upper portion of the container contains a lower concentration of the particles. Compositions that contain only dissolved chemicals may also experience similar stratification. In the instance of stratification or settling of dissolved or suspended materials of a slurry or liquid (e.g., solution) useful in microelectronic processing, e.g., chemical-mechanical processing and related cleaning or treatment steps, the slurry or liquid requires re-homogenized before use in a tool or processing system. During use, in fact, over a period of hours, days, or weeks during which a slurry or liquid is removed from a container for use, a common practice is to mix the liquid on a continual, un-interrupted basis, so that the liquid remains homogenous for use on demand.
Typically, a slurry or other process composition for a CMP process is withdrawn from a container such as a drum or a tote using an inserted dip tube that is connected to a dispense head that is attached to a feed line and a distribution system of a CMP processing tool. The feed dip tube extends to near the bottom of the container, so that a process composition (e.g., slurry, cleaning composition) is withdrawn from that location first. Without re-homogenizing a stratified CMP slurry, the slurry may have a high concentration of its solid abrasive materials at the bottom of the container, and a lower concentration at the top of the container, due to stratification. The result is inconsistent concentrations of the abrasive particles in slurry removed from the same container. Slurry that is removed first, from the bottom of the drum, will have a relatively higher solids concentration as compared to the solids concentrations of slurry removed last. The concentration of the abrasive materials in the slurry will become progressively lower as the slurry is emptied from the container. The variation in solids gives a “saw tooth” result when charted over a series of drums that contain stratified slurry, with increases in concentration occurring at every change from an empty container to a full container.
For CMP applications, at least two types of mechanical mixing (i.e., “agitation”) may be useful: constant speed mixers, and variable speed mixers. Constant speed mixers have drawbacks of the mixer potentially causing splashing or a vortex in the liquid as an amount of liquid in a container gradually decreases. As an amount of liquid in a container is gradually reduced, the rate of mixing (if constant) becomes too high for the amount of the remaining liquid and the result can be splashing, vortex formation, or foam formation.
Variable-speed mixing to control a mixer speed as liquid is removed from a container has been proposed as a way to reduce splashing, vortex formation, or foaming. Capacitive sensors have been used to detect the level of liquid in a container, but these types of sensors have significant drawbacks. If foam becomes present at a surface of a liquid inside of the container, the foam may often cause the capacitive sensor to signal an incorrect reading, which can cause an incorrect mixing speed relative to a correct level of liquid in the container. Additionally, capacitive sensors do not perform adequately with respect to monitoring an entire volume of a container, because, for example, these types of sensors may not allow for detection of amounts of liquid in a container at a bottom portion of the container. Capacitive sensors are also only a pinpoint detection source only allowing detection at set points.
The mixing of liquids in other areas of technology can also suffer undesired mixing effects if mixing is not sufficiently well controlled. Some types of chemical liquids, e.g., certain reactive monomers and polymers, can suffer premature and undesired reaction activity in the presence of air. In these systems, a vortex that causes air to become incorporated into the liquid as the liquid is being mixed can be highly detrimental to the quality or consistency of the liquid. Controlled mixing is desired to prevent a vortex, which may bring air into the liquid.
In various other liquid chemical systems, bubbles in the liquid are undesired, for a range of reasons. In these systems, controlled mixing can also be desired to prevent a vortex that would incorporate bubbles into the liquid.
Variable speed mixers have been an option to try to control mixing speed of mixing systems. However, improved methods of using mixing systems to prevent undesired mixing effects continue to be desired and of commercial value and importance.
The ability to effect uniform mixing, homogenization, or re-homogenization of a liquid composition in a container, in a commercially efficient manner, while liquid is slowly or gradually, optionally intermittently, removed from or added to the container, and without undesired mixing effects, has a high level of value for a wide range of commercial, industrial, and institutional mixing applications.
For countless varieties of different types of liquid materials, use or processing of the liquid requires mixing, homogenizing, re-homogenizing, or continuously circulating the liquid within a container during a period of use or processing of the liquid. Often, during a period of use or processing, liquid will be continuously or intermittently removed from or added to the container. If mixing during a period of use or processing is performed with insufficient care and attention, especially for a system that involves adding liquid to or removing liquid from the container during mixing, the mixing can result in undesired mixing effects. As the amount of liquid in the container is increased or decreased, if mixing speed is not adjusted, a result can be an effect such as one or more of: splashing at the surface of the liquid being mixed, the formation of foam or bubbles in the surface of the liquid, or the formation of a vortex of the liquid in the container during mixing.
A vortex is generally undesired during mixing of many liquids, because a vortex may detrimentally incorporate bubbles of gas present above the liquid (e.g., air), into the liquid. Generally, bubbles of air incorporated into a liquid may be potentially detrimental to the liquid or to a process of subsequently using of the liquid, depending on the type of the liquid and its purpose. Air bubbles in a liquid may be detrimental to the liquid if the liquid is sensitive to air, e.g., the liquid contains a chemical that reacts or degrades in the presence of air or a constituent of air such as oxygen or moisture. Alternately, certain liquids are desirably mixed without allowing air to be entrapped in the liquid in the form of bubbles, to prevent the bubbles from being present in a subsequent use or in a product made from the liquid. As one example, the formation of bubbles during mixing of a curable liquid is preferably avoided, if the cured liquid is one that should not contain bubbles.
Splashing of a liquid during mixing in a manner that causes small or tiny droplets of the liquid to contact a sidewall of a container that holds the liquid, can also be detrimental when mixing certain types of liquids. For some liquids, splashing might be considered an inconsequential occurrence and may not have a significant detrimental effect during mixing of the liquid or during use of the liquid after mixing. But splashing may have a noticeable detrimental effect when mixing a liquid that contains a very precise (especially also, very low) amount of dissolved chemical ingredient. In specific, splashing of a liquid can cause a transfer of a dissolved chemical ingredient from the liquid to the interior sidewall of the container. Upon splashing of liquid droplets onto sidewalls at an interior of a container, the splashed droplets become dried upon the surface, leaving the dissolved chemical ingredient from that drop as dried material on the sidewall. In the end, the dissolved chemical material of the splashed droplet has been removed from the liquid in the container. The concentration of that dissolved chemical ingredient in the remaining liquid of the container, when dispensed from the container, will be reduced relative to a desired concentration, i.e., relative to an original concentration of that dissolved chemical ingredient in the liquid when placed in the container, and when the container was full. In certain processing systems, a very small loss of dissolved chemical ingredient from a liquid raw material, caused by this form of splashing, can be sufficient to produce undesired and detrimental effects in a process that uses the liquid raw material.
Foaming or bubble formation of liquid during mixing can also be detrimental to a liquid. On one respect, for certain types of liquids, foaming (as with splashing) can cause a loss of a dissolved chemical ingredient from the liquid due to the foam contacting sidewalls of the container and drying on the sidewalls to leave the dissolved chemical ingredient at the sidewall, removed from the liquid. The result is a reduced concentration of that chemical ingredient in the liquid when the liquid is eventually dispensed from the container for use, especially for a final remaining portion of the liquid removed from the container after most of the liquid has been previously removed.
Additionally, foam is prone to drying, and foam of certain types of liquids, e.g., a CMP slurry, which contains solid abrasive particles, can cause drying of the abrasive particles within the container. The dried abrasive particles can agglomerate to form agglomerated particles that are capable of causing unwanted processing difficulties in a system that uses the liquid, such as clogging of a filter or a dispense head, or defects in a workpiece being processed by use of the slurry.